Eccentrically loaded structural members and methods of forming the same

ABSTRACT

Eccentrically loaded structural members and methods of forming the same. The structural members have their compressive loading axes offset from their load central longitudinal axes.

CROSS-REFERENCE TO RELATED APPLICATION

This is a divisional application of U.S. application Ser. No.14/250,309, filed Apr. 10, 2014, entitled “ECCENTRICALLY LOADEDSTRUCTURAL MEMBERS AND METHODS OF FORMING THE SAME”, which claims thepriority to and the benefit of U.S. Provisional Application Ser. No.61/810,653, filed Apr. 10, 2013, entitled “ECCENTRICALLY LOADEDSTRUCTURAL MEMBERS AND PROCESSES”, the entire contents of which areincorporated by reference.

BACKGROUND

Fuse structural members are elongated members which must fail atpredetermined loads to avoid damage to surrounding mechanical elements,while still maintaining sufficient strength and rigidity to efficientlyand safely transmit forces at sub-fuse loads. In compression, thesuccessful design of a fuse load is complicated by the variable degreein bending of the member as the load increases. The failure point anddirection of failure is driven by the presence of manufacturingeccentricities and tolerances, material variations and cross sectionalshape throughout the length of the member.

Fuse loads are the predetermined loads that cause fuse structuralmembers to fail prior to damaging surrounding elements. Fuse loads incompression are usually accomplished by the use of either a shear pin orspring mechanism. In the case of the shear pin the load that causes thepin to shear in compression will also shear at the same load in tension.In the case of a spring, the assembly does not fail but begins todisplace according to Hooks Law once the spring preload is surpassed.Therefore, the spring continues to add force as displacement increases.Both shear pin and spring mechanism fuse loads are expensive tomanufacture, add complexity to designs and have limitations to theirapplications. In weight or fatigue critical applications, such asaerospace, the use of a spring mechanism may be impractical.

Typical rod assembly design for compression relies heavily on Euler andJohnson-Euler equations to determine safe working loads. These equationsestablish the theoretical lowest compressive loads where columns, as forexample structural rod assemblies and struts, can be expected to buckle.A theoretical column with no load eccentricity, subject to an increasingcompressive force, would eventually fail due to compressive yielding.This compressive yielding creates a local instability that willeventually lead to bending and subsequent buckling. Initial yieldingfailure would occur at an unpredictable location and cause instabilityin an unpredictable direction.

Structural members that experience significant bending duringcompressive loads may gain structural support if they rest againstneighboring elements. According to Euler compressive column theory, acolumn 10 that gains support 12 (FIG. 1b ) at mid span will have abuckling load 8 times higher than column 10 supported by pins 14 (FIG.1c ). A column 10 that gains transverse support at its ends 16 bysupport members 18 (FIG. 1a ) will have a buckling load 4 times higherthan a column supported only by pins (FIG. 1c ). Even slight supportgained from surrounding elements can have large impacts on the magnitudeof the expected compressive failure load.

Additionally, failures in pure compression without significant bendingrely heavily on compressive yielding. Compressive yielding and theresulting plasticity are extremely hard to accurately predict and can beaffected by many factors which are difficult to consistently control ina manufacturing environment. Euler bending failures are described asgeometric failures and are not dependent upon the stress capacity of thematerial.

SUMMARY OF THE INVENTION

In an example embodiment, an elongated member is provided. The elongatedmember has an elongated body having a load central longitudinal axis. Afirst fitting is coupled to a first end of the elongated body forapplying a compressive load to the elongated body along a first axisoffset from said load central longitudinal axis. A second fitting iscoupled to a second end of the elongated body opposite the first end forapplying a compressive load to the elongated body along a second axisoffset from the load central longitudinal axis.

${The}\mspace{14mu}{elongated}\mspace{14mu}{body}\mspace{11mu}\frac{\frac{L_{e}}{r_{\min}}}{\sqrt{\frac{\pi^{2}E}{\sigma_{y}}}}$is greater or equal to 0.5, wherein:

-   -   L=Length of the elongated body    -   L_(e)=Effective Length of the elongated body    -   I_(min)=Minimum Second Moment of Inertia of the elongated body    -   A=Cross sectional Area of the elongated body    -   r_(min)=Minimum Radius of Gyration of

${{the}\mspace{14mu}{elongated}\mspace{14mu}{body}} = \sqrt{\frac{I_{\min}}{A}}$

-   -   σ_(y)=Yield Stress of the elongated body    -   E=Modulus of Elasticity of the elongated body.

In another example embodiment, another elongated member is provided. Theelongated member includes an elongated body having an outer surfacedefined about a central longitudinal axis. The outer surface defines afirst recess section about at least a portion of the body. A firstfitting is coupled to a first end of the elongated body for applying acompressive load to the elongated body along the central longitudinalaxis. A second fitting coupled to a second end of the elongated bodyopposite the first end for applying a compressive load to the elongatedbody along said central longitudinal axis.

${The}\mspace{14mu}{said}\mspace{14mu}{elongated}\mspace{14mu}{body}\mspace{11mu}\frac{\frac{L_{e}}{r_{\min}}}{\sqrt{\frac{\pi^{2}E}{\sigma_{y}}}}$is greater or equal to 0.5,wherein:

-   -   L=Length of the elongated body    -   L_(e)=Effective Length of the elongated body    -   I_(min)=Minimum Second Moment of Inertia of the elongated body    -   A=Cross sectional Area of the elongated body    -   r_(min)=Minimum Radius of Gyration of

${{the}\mspace{14mu}{elongated}\mspace{14mu}{body}} = \sqrt{\frac{I_{\min}}{A}}$

-   -   σ_(y)=Yield Stress of the elongated body    -   E=Modulus of Elasticity of the elongated body.

In yet another example embodiment, a further elongated member isprovided. The elongated member includes an elongated body having anouter surface about a central longitudinal axis. The elongated body isbent defining having an arcuate shape. A first fitting is coupled to afirst end of the elongated body for applying a compressive load to theelongated body. A second fitting is coupled to a second end of theelongated body opposite the first end for applying a compressive load tothe elongated body.

${The}\mspace{14mu}{elongated}\mspace{14mu}{body}\mspace{11mu}\frac{\frac{L_{e}}{r_{\min}}}{\sqrt{\frac{\pi^{2}E}{\sigma_{y}}}}$is greater or equal to 0.5,wherein:

-   -   L=Length of the elongated body    -   L_(e)=Effective Length of the elongated body    -   I_(min)=Minimum Second Moment of Inertia of the elongated body    -   A=Cross sectional Area of the elongated body    -   r_(min)=Minimum Radius of Gyration of

${{the}\mspace{14mu}{elongated}\mspace{14mu}{body}} = \sqrt{\frac{I_{\min}}{A}}$

-   -   σ_(y)=Yield Stress of the elongated body    -   E=Modulus of Elasticity of the elongated body.

In yet a further example embodiment, a method of tuning a structuralmember, including an eccentricity, to a fuse load is provided. Themethod includes subjecting the structural member to a compressive axialload by compressive axial displacement, and stopping the compressiveaxial displacement when the compressive axial load drops to a level ofthe desired fuse load. The structural member is tuned to the fuse loadwhen the compressive axial load has plastically deformed the structuralmember.

In another example embodiment, a method of tuning a structural member toa fuse load is provided. The method includes subjecting the structuralmember to a side load sufficient to bend the structural member in adesired direction, subjecting the structural member to a compressiveaxial load by compressive axial displacement, stopping the compressiveaxial load, holding the axial displacement, and removing the side load,when the structural member has plastically deformed at mid-span orproximate mid-span. The method also includes continuing to subject thestructural member to a compressive axial load by compressive axialdisplacement, and to stopping the compressive axial displacement whenthe compressive axial load drops to a level of the desired fuse load.The structural member is tuned to the fuse load when the compressiveaxial load has plastically deformed the structural member.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a, 1b and 1c are schematic side views depicting different columnsunder compressive loading.

FIG. 2a is a side view of an example embodiment structural memberincluding end fittings.

FIG. 2b is an end view of the structural member depicted in FIG. 2a witha fitting.

FIG. 3a is a side view of another example structural member withoutfittings.

FIGS. 3b and 3c are an end view and a side view of an example embodimentinsert for mounting on the structural member shown in FIG. 3 a.

FIG. 3d is a side view of another example structural member.

FIG. 4 is an end view of another example embodiment insert.

FIG. 5 is a partial side view of an example embodiment structural memberincluding an example embodiment fitting.

FIG. 6 is a partial side view of another example embodiment structuralmember with an integral fitting.

FIG. 7 is a side view of another example embodiment structural memberincluding fittings.

FIG. 8 is a side view of another example embodiment structural memberwithout fittings.

FIGS. 9a and 9b are side views at 90° to each other of another exampleembodiment structural member.

FIGS. 10a and 10b are side views at 90° to each other of yet anotherexample embodiment structural member.

FIGS. 11a and 11b are side views at 90° to each other of a furtherexample embodiment structural member.

FIGS. 12a and 12b are side views at 90° to each other of yet a furtherexample embodiment structural member.

FIGS. 13 and 14 are graphs of load vs displacement data from straighttubes with 0.015 inch and 0.030 inch eccentricity, respectively.

FIG. 15 is a flow chart of an example embodiment approach for tuning afuse load of an example embodiment structural member with eccentricity.

FIG. 16 is a flow chart of an example embodiment approach for tuning afuse load of a structural member without eccentricity or with justmanufacturing eccentricity.

DETAILED DESCRIPTION

Applicants have developed fuse elongated structural member embodimentswhich greatly increase the repeatability and reliability of thecompressive failure load and the resulting direction of structuralmember deformation during bending and subsequent failure.

Through the addition of purposeful eccentric loading, a moment isintroduced throughout the member, which results in bending in apredetermined direction. By being able to control the direction of thebending and subsequent failure, the structural member allows for bendingthat is unobstructed along its full length, thus avoiding undesirablecolumn fixity conditions. Sufficient load eccentricity also acts toensure failure in high bending or Euler failure, which is moreconsistent. Applicants have discovered that by introducing sufficientload eccentricity above and beyond the manufacturing eccentricity, i.e.,the eccentricity introduced during manufacturing due to manufacturingimperfections, they were able to control and predict compressive failureload (i.e., the fuse load) within 10% of the designed failure load, insome example embodiments, within 5% of the designed failure load, and infurther example embodiments within 1% of designed failure load. A “loadcentral longitudinal axis” as used herein refers to an axis of anelongated structure where when the structure is loaded under compressionalong this load central longitudinal axis the structure will crumple andfail at the compressive yield strength of the material from which it ismade. In elongated structures such as cylinders having a centrallongitudinal axis which are manufactured without any eccentricity, theload central longitudinal axis is the same as the structures centrallongitudinal axis. Most structures have some manufacturing eccentricitybuilt in (i.e., eccentricity caused by manufacturing imperfections) andas a result a compressive load applied along its central longitudinalaxis leads to buckling of the structure at a compressive load lower thanthe compressive yield strength of the material that the structure ismade of.

Fuse loads which are higher in tension than in compression can also beaccommodated by incorporation of a shear pin for the tensile fuse loadand eccentric bending for the compressive fuse load. The embodiments ofthis disclosure, offer a less expensive and more efficient means toachieve fuse loads in compression while still allowing for tensile fuseloads at a greater force if required.

Load eccentricity can be introduced in several ways. In exampleembodiments, load eccentricity is introduced by offset of connectingthreads between end fitting and structural member, offset of internalthreads of a structural member insert, offset of pin connections in endfitting threaded to or otherwise attached to structural member, oroffset of pin connections in structural member; or pre-curved orpre-bent structural member. The eccentricity of the example embodimentsstructures is such that the compressive loading axis, i.e., the axisalong which the load is applied is offset from the structure's centrallongitudinal axis when the structure has no eccentricity and is offsetfrom the load central axis when structure has manufacturingeccentricity.

Structural members where

$\frac{L_{e}}{r_{\min}} > \sqrt{\frac{\pi^{2}E}{\sigma_{y}}}$are considered long and slender by Euler definition, where,L=LengthL_(e)=Effective Length (varies by end fixity condition)

-   -   An infinite number of effective lengths exist for any given        Length due to variations in fixity and loading conditions.    -   Examples of some common effective lengths with concentrated        axial loading:

Pinned-Pinned; L_(e) = L Fixed-Free; L_(e) = 2 L${{Fixed}\text{-}{Fixed}};{L_{e} = {\frac{1}{2}L}}$${{Fixed}\text{-}{Pinned}};{L_{e} = {\frac{7}{10}L}}$

-   -   (Common effective lengths and methods of calculating less common        effective lengths for various loading and boundary conditions of        structures is provided in Jones, Robert M. Buckling of Bars,        Plates, and Shells. Blacksburg, Va.; Bull Ridge Publishing,        2006, the contents of which are fully incorporated herein by        reference.)        I_(min)=Minimum Second Moment of Inertia        A=Cross sectional Area        r_(min)=Minimum Radius of

$\left( \frac{L_{e}}{r_{\min}} \right)$σ_(y)=Yield StressE=Modulus of Elasticity

A factor common to example embodiments detailed in this disclosure isthe slenderness ratio

${Gyration} = \sqrt{\frac{I_{\min}}{A}}$of the structural member. This dictates the member's propensity to bendwhile under compression. Members with slenderness ratios high enough tobe considered long and slender under Euler definition require lower loadeccentricities to ensure predictable bending. Designs become lesseffective as slenderness ratios decrease and required eccentricities toensure bending increase.

“Eccentricity”, “load eccentricity” or “loading eccentricity” as usedherein is the maximum distance between the structure's centrallongitudinal axis and the compression load axis as measuredperpendicularly from the central longitudinal axis along the body of thestructural member.

In one example embodiment, a first fuse member (i.e., structural member)21 has long slender tube 10 by Euler definition having eccentric bore 20for accepting a fitting 23 into said eccentric bore on one or both ends22, as for example shown in FIGS. 2a and 2b . The eccentric end bore 20has a central longitudinal axis 39 offset from a central longitudinalaxis 35 of the structural member 21. The loading to the fuse member isapplied through the fittings. In an example embodiment, each eccentricbore 20 has internal threads 24 onto which is threaded a correspondingfitting. Eccentric threading of tube is formed, machined or tappedoff-center from the axis of the tube 10. In an example embodiment, onefitting is threaded in a clockwise direction, while the other fitting isthreaded in a counter-clockwise direction. In this regard, rotating thetube in one direction while holding the fittings, will cause thefittings to thread into their corresponding bores, while rotating thetube in the opposite direction, while holding the fittings will causethe fittings to unthread from their corresponding bores. Once threadedsufficiently, each fitting may be locked in place with a correspondinglock nut which is threaded onto the threads of the fitting and fastenedagainst the corresponding end of the tube. In an example embodiment eachfitting has an opening 27 through which a load will be applied and ashaft 29 which is received in the end bore of the tube. In exampleembodiments, the load is applied by a pin inserted in the opening 27.Structural members which are loaded via pins or other members insertedinto the fitting openings 27 are sometimes referred to as “pin-to-pinconnected structural members”. The opening 27 has a center 31 in linewith the longitudinal central axis 33 of the shaft 29. The centrallongitudinal axis 33 of the shaft is co-axial with the centrallongitudinal axis 39 of the end bore. The distance 37 between thecentral longitudinal axis 35 and the central longitudinal axis 33 of theend bore as measured perpendicularly from the central longitudinal axis37 is the loading eccentricity.

For offset of threads in an example embodiment, enough material shouldbe present at the connection interface of the end fitting and the tubebody to allow for both the full formation of threads and sufficientmaterial wall thickness to cope with the additional stresses and momentsimparted by the eccentric loading.

In another example embodiment, a long slender tube 32, by Eulerdefinition, has thin wall geometry and concentric end bores 32 (FIG. 3a) for receiving inserts 34 (FIGS. 3b and 3c ). In an example embodiment,the end bores 32 are threaded, i.e., they have an internal threadedsurface 36 for receiving a threaded portion 38 of a corresponding insert34. In example embodiment, at least one insert 34 is received into oneend bore. The inserts 34 have an eccentrically formed hole 40. In anexample embodiment, the eccentrically formed hole is formed on theinsert off-center. In an example embodiment, the off-center hole 40 ineach insert is formed by machining. In an example embodiment, theeccentrically formed hole has internal threads 42 for receiving threadsof a fitting. In an example embodiment, where an insert is mated, e.g.,threaded on each of the end bores 30, the eccentrically formed holes 40on each insert are aligned, i.e., the have the same central axis. Toassist with the alignment, in an example embodiment, a keyway slot 44 isformed on each insert end face 46. In an example embodiment, theeccentrically formed hole 40 has a central axis 50 offset from thecentral axis 52 of the insert by a distance 54, such that a diameter 56of the eccentrically formed hole is offset in parallel from a diameter58 of the insert by the same distance. In such example embodiment, eachkeyway slot is aligned along the diameter 56 of the eccentrically formedhole 40. In this regard once the inserts 34 are fitted into theircorresponding end bores, the keyway slot of one insert is aligned to beparallel and on the same side of the long slender tube load central axiswith the keyway slot of the other insert, and the inserts are orientedlocked in place as for example by using wrenching flats formed on anouter surface of the insert or a nut also threaded on the outer surfaceof the insert. An adhesive may be used to adhere the insert to the longslender tube. In an example embodiment, once both inserts are orientedwith their keyway slots parallel, the eccentrically formed holes of bothinserts are coaxially aligned and then held through swaging of the tubebody material against the threads of the insert, thus locking theinsert's position and orientation with the tube body. To further helpwith the alignment, in an example embodiment, one fitting is threaded ina clockwise direction, while the other fitting is threaded in acounter-clockwise direction into their corresponding inserts. In thisregard, rotating the tube in one direction while holding the fittings,will cause the fittings to thread into their corresponding bores, whilerotating the tube in the opposite direction, while holding the fittingswill cause the fittings to unthread from their corresponding bores.Orientation of the end fitting is achieved through the use of a lockingdevice which locks the keyway of the insert at the same orientation ofthe keyway on the end fitting. Upon proper orientation of the endfittings to the insert and thus the tube, a lock nut is tightened intoplace which locks the position of the end fittings to the tube and eachother. In an example embodiment each fitting has an opening 27 throughwhich a load will be applied and a shaft 29 which is received in the endbore of the tube. The opening 27 has a center 31 in line with alongitudinal central axis 33 of the shaft 29. With these exampleembodiments, the distance 54 is the loading eccentricity.

In a further example embodiment, an outer diameter of the tube isreduced at mid-span 60 so as to lower the strength of the tube atmid-span and thus adjusting the failure point of the desired fuse load(FIG. 3d ).

For structural members with insufficient material at the tube ends,inserts 62 may be used to strengthen the area while providing more areafor eccentrically located threads (FIG. 4).

In a further example embodiment, a structural member, such as a tube orstrut 70 is used with concentric end bores for receiving fittings, eachfitting 72 having an off-centered opening 74, as for example shown inFIG. 5. The opening 74 receives the load applying member as for examplea bearing cartridge, or a pin, or a bushing or a spherical ball. In anexample embodiment, the axes 76 of the end fitting openings 74 through adiameter or a center of the openings and are offset by a distance 78from the central axis 80 of the structural member 70. In an exampleembodiment, the axes of the openings 74 are aligned with each other. Inother example embodiments the fitting(s) 72 is integrally formed withthe structural member 70, as for example shown in FIG. 6. The distance75 is the loading eccentricity.

In another example embodiment as shown in FIG. 7, a structural member70, such as a tube or strut is formed in into a bent shape or ispre-bent into a bent arcuate shape where a mid-span section 82 of thetube is offset from the end sections or extremities 84. In other words,a plane 86 through said mid-section center which is parallel to a plane88 through the centers of the fitting openings 27, is spaced a greaterdistance from the plane 88 than a plane 90 through the centers of theends 91 of the structural member. The plane 86 is sufficiently offsetfrom the plane 88 by a distance 84 to induce desired bending and apredictable failure load when under compressive loading via thefittings. The distance 76 defines the loading eccentricity. In anexample embodiment, the structural member is plastically deformed at orproximate its mid-span.

In yet another example embodiment, a structural member, such as a tubeor strut has a straight mid-section 92 as for example shown in FIG. 8and has swaged end sections 94 ending in end sections 95, each definingan end bore 96 for receiving a fitting. In an example embodiment, themid-section is a cylindrical member having a central longitudinal axis98, and each end section is also cylindrical having a centrallongitudinal axis 100. In the example embodiment both end sections havethe same central longitudinal axis 100 which is offset in parallel by adistance 99 from the mid-section central longitudinal axis 98. Thedistance 99 is the loading eccentricity.

In yet a further example embodiment, a structural member, such as a tubeor strut 110 is formed, as for example by machining, or by othermethods, to have a second lower moment of inertia about across-sectional axis in line with the desired axis of bending 112 (FIGS.9a and 10a ) and a higher second moment of inertia about thecross-sectional axis 114 which is perpendicular to the desired axis ofbending 112 (FIGS. 9b and 10b ). This, in one example embodiment asshown in FIGS. 9a, 9b , 10 a, and 10 b, is accomplished by making thestructural member thinner at its mid-section 116 when viewed along thedesired bending axis 112 as shown in FIGS. 9b and 10b and thicker whenviewed along cross-sectional axis 114 as shown in FIGS. 10a and 10b . Inthe embodiments shown in FIGS. 9b and 10b , the mid-section 116 isthinner than the end sections 118. In the embodiments shown in FIGS. 9a,9b, 10a and 10b , the central longitudinal axis 120 is co-linear withthe central longitudinal axis 122 of the end sections.

In other example embodiments as shown in FIGS. 11a, 11b . 12 a and 12 b,eccentric loading is allowed by machining or forming the structuralmember (e.g., the tube or strut) 110, by removing material or byrecessing the member at one side of the mid-section 116 defining arecessed surface 130. Thus, a load central longitudinal axis 133 of therecessed structure is offset by a distance 135 from load axis 132. Inthis regard a longitudinal axis 132 along which a load will be appliedis closer to the recessed surface than to surface 134 opposite therecessed surface as measured along a direction 136 through andperpendicular the longitudinal axis 132. The distance 135 is the loadingeccentricity. In this regard, structural member will bend toward therecessed section when under compression.

In the example embodiments, including the recessed mid-section, thenon-recessed portion of the mid-section in example embodiments iscircular in cross-section. In other embodiments such non-recessedsection may have other cross-sectional shapes as for example polygonal,elliptical, etc. The example embodiments of FIGS. 2a to 8 and FIGS. 11ato 12b loading eccentricity is provided that exceeds any manufacturingeccentricity present and/or is such as to cause bending in a directionother than the direction the structure would have failed undercompressive loading due to its manufacturing eccentricity.

Pin-to-pin connected structural members are designed and manufacturedwith the least amount of load eccentricity possible. Structural membersthat incorporate the eccentric loading as in the example embodiments ofthe present disclosure allow for predictable failure loads anddirections of failure. This enables the design of structural membersthat are able to fail at highly consistent loads.

In the example embodiments provided herein, the load is applied along aload axis that is offset from a central longitudinal axis of theelongated slender member. This load axis may be parallel to the centrallongitudinal axis of the member. In some embodiments, this load axis isnot parallel to the central longitudinal axis but is offset at each endof the structural member by the same distance from the centrallongitudinal axis. In other example embodiments, the elongated slendermember may not be cylindrical and may have other shapes, as for example,it may be an I-beam in cross-section, which is symmetric about asymmetry plane through its central longitudinal axis. In the embodimentswhere a section of the central longitudinal member is removed orrecessed, as for example shown in FIGS. 9a to 11b , the structuralmember may not be symmetrical about such a symmetry plane.

In example embodiments, by ensuring the direction of the failure andthat the end connections will remain free to rotate, moments are nottransmitted through the structural member, e,g, the tube or strut, tothe next connecting element.

Example structural members having on load eccentricity induced bendingare able to have higher fuse loads in tension than in compression. Incompression the fuse load is driven by the bending failure and intension by the weakest cross section along the member.

Testing has been conducted on the induced eccentricity concept byoffsetting threads in the example embodiment, as shown in FIG. 2a .Tests were performed using 0.015 in. and 0.030 in. thread offset on0.500 inch outside diameter (OD)×0.156 inch thick wall stock tube. FIGS.13 and 14 show the results of this testing. For the 0.015 incheccentrically loaded thread specimen, significant scatter in the failureloads can be seen. Manufacturing eccentricities in the aerospaceindustry vary by design and scale of the structural member. Typically,structural members designed for compressive loads have manufacturingeccentricity held to the tightest limits possible, usually well under0.015 inch for lengths up to approximately 24 inch As assembliesincrease in length, some manufacturing eccentricities may increase dueto mid-span run out at a rate of approximately 0.0005 inch/inch oflength. As can be seen from FIG. 10, for the higher load failures, loadvs. axial deflection remained linear as load increased until localizedcompressive yielding was followed by an abrupt failure. Exampleembodiment elongated structures have an eccentricity rate equal to themanufacturing eccentricity rate (R) plus an additional eccentricityrate. In one example embodiment, the additional eccentricity rate is at0.0001 inch/inch of length. In another example embodiment, it is 0.0002inch/inch of length. In a further example embodiment, it is 0.0003inch/inch of length. In a yet another example embodiment, it is at least0.0004 inch/inch of length. In yet a further example embodiment, it isat least 0.0005 inch/inch of length.

In the testing samples that failed at lower loads, an abrupt change fromlinear axial deformation to bending can be seen followed by a gradualincrease in the rate of deflection, which is a result of an increasingbending rate. All tubes in these tests did bend in the predicteddirection; however the consistent nature of an Euler failure, with largebending deflections, was not consistently present. The mixture offailure types as previously discussed was still present with the 0.015in. eccentricity and lead to a large compressive failure load scatterbetween the testing samples.

Testing conducted on the 0.030 inch eccentrically located threads showeda consistent failure in bending that was both repeatable and reliable,as can be seen in FIG. 11. The significant bending can be seen in FIG.11 as the point where load vs. displacement is no longer linear. Worthyof note with this set of testing specimens is the more gradual entryinto bending and a highly repeatable and reliable failure load.

While the example embodiments have been described with elongatedstructures that are long and slender by Euler definition, Applicantshave discovered that they can obtain similar or the same fuse loadpredictability with structural members of intermediate slendernessratios where

$1 \geq \frac{\frac{L_{e}}{r_{\min}}}{\sqrt{\frac{\pi^{2}E}{\sigma_{y}}}} \geq \frac{1}{2}$Although such structural members may require greater inducedeccentricity as compared to that required to overcome the manufacturingeccentricity alone. Thus, in an example embodiment, the slendernessratio of the structural member is equal to or greater than

$\frac{1}{2}\mspace{14mu}{\left( {{i.e.},{\frac{\frac{L_{e}}{r_{\min}}}{\sqrt{\frac{\pi^{2}E}{\sigma_{y}}}} \geq \frac{1}{2}}} \right).}$

Applicants have discovered that the minimum eccentricity required toinduce bending in example embodiments in a predictable direction and atconsistent failure loads can be defined by the following equation.

${{{Min}.\mspace{14mu}{Req}.\mspace{14mu}{Eccentricity}} = {\left( {{.02}\mspace{11mu}\frac{\sqrt{\frac{\pi^{2}E}{\sigma_{y}}}}{\left( \frac{L_{e}}{r_{\min}} \right)}} \right) + R}};$${When}\mspace{14mu}\left( {\frac{1}{2} \leq \frac{\frac{L_{e}}{r_{\min}}}{\sqrt{\frac{\pi^{2}E}{\sigma_{y}}}}} \right)\mspace{14mu}{and}\mspace{14mu}\left( {{3\mspace{14mu}{in}} \leq L \geq {72\mspace{14mu}{in}}} \right)$where R is the manufacturing eccentricity rate. In an exampleembodiment, R is 0.0005 or less. For intermediate structural memberswhere

${\frac{1}{2} \leq \frac{\frac{L_{e}}{r_{\min}}}{\sqrt{\frac{\pi^{2}E}{\sigma_{y}}}} \leq 1},$

${{{Min}.\mspace{14mu}{Req}.\mspace{14mu}{Eccentricity}} = \left( {{.02}\;\frac{\sqrt{\frac{\pi^{2}E}{\sigma_{y}}}}{\left( \frac{L_{e}}{r_{\min}} \right)}} \right)};$${When}\mspace{14mu}\left( {\frac{1}{2} \leq \frac{\frac{L_{e}}{r_{\min}}}{\sqrt{\frac{\pi^{2}E}{\sigma_{y}}}} \leq 1} \right)\mspace{14mu}{and}\mspace{14mu}\left( {{3\mspace{14mu}{in}} \leq L \geq {72\mspace{14mu}{in}}} \right)$In any of the aforementioned embodiments, the compression load axis(i.e., the axis along which the compressive load is applied) is parallelwith the load central longitudinal axis.

Applicants have also discovered that they can tune the exampleembodiment structural members to fail at a desired fuse load. To do so,Applicants first subject the structural member to a compressive axialload via controlled displacement (block 152, FIG. 15) while the ends ofthe structural member are constrained as they would be constrainedduring actual use. For example, in many aircraft embodiments, thestructural members are pin to pin constrained. In other words, the endsof the structural member are rotatable about their corresponding pins towhich they are coupled. By subjecting the structural member to acompressive axial load by displacement in the compressive direction, theload would reach an ultimate strength, and the load would then begin todrop upon further displacement. If the axial load drops to a level of adesired fuse load (block 154), then the axial displacement is stopped(block 156). If not, the axial displacement continues until the axialload is dropped to the level of a desired fuse load (block 154). Oncethe axial load is stopped, the axial load is removed (block 158), thestructural member is evaluated to confirm that it is plasticallydeformed (block 160). Many methods well known in the art can be used toconfirm plastic formation. One example method may be to inspect the bentstructural member to see if it has stayed in the bent state. That wouldbe an indication of plastic deformation. If plastically deformed, thenthe structural member is tuned to the desired fuse load and should failat or slightly above such fuse load. If the structural member is notplastically deformed, then the structural member needs to be redesigned(e.g. the eccentricity or slenderness of the structural member may needto be modified) (block 161).

Applicants have also discovered that they can tune the fuse load evenfor structural members which are not eccentrically loaded or which onlyhave a manufacturing eccentricity built into them. In such case, thestructural member is subjected to a side load sufficient to overcome themanufacturing eccentricities and induce bending to a desired direction(block 162, FIG. 16) while constrained as they would during actual use.The structural member is then subjected to an increase in compressiveaxial load by controlled compressive displacement (block 164). Duringsuch displacement, the load would reach an ultimate strength and theload would then begin to drop upon further displacement. Thedisplacement is continued until the structural member is plasticallydeformed at its mid-span (block 166). As discussed previously, plasticdeformation can be easily determined using well known methods. If thestructural member is plastically deformed, then the loading is stoppedand the axial displacement is held (block 168). The side load is thenremoved (block 170) and the axial displacement is then resumed in thecompressive direction (block 172). If the load continues to decrease(block 174), then the axial displacement is continually applied untilthe axial compressive load drops to a level that is equal to the desiredfuse load (block 176). When at that level, the axial displacement isstopped (block 178) and the axial load is removed (block 180). If themember has further plastically deformed (block 182) over the plasticdeformation as per block 166, then the member is tuned to the desiredfuse load. If not, then the structural member needs to be redesigned(e.g. the eccentricity or slenderness of the structural member may needto be modified). If the applied load is not decreasing (block 174) thenthe structural member is continuously subjected to an increase incompressive loading by controlled displacement (block 164) until themember is plastically deformed at the mid-span (block 166). If the sideload has already been removed (block 184), then the axial displacementis continued until the axial compressive load drops to a level equal tothe desired fuse load level (block 176). In such case, the axialdisplacement is stopped (block 178) and the load is removed (block 180)and the further plastic deformation is confirmed (block 182). If theplastic deformation is confirmed, the structural member is now tuned tothe desired fuse load. If not, then the structural member needs to beredesigned (block 184).

Applicants have discovered that they can design elongated structuralmembers to bend in, or generally in, a predetermined direction and tofail at a, or generally at a, predetermined load by incorporatingsufficient eccentricity to such members. This is especially importantwhen such structural members are used in aircraft where unpredictedfailure as well as unpredicted bending direction can cause damage to andor jam surrounding structures and parts. Thus, in example embodiments, amethod is provided for designing or forming elongated structural memberthat fail at a, or generally at a, predetermined load and in, orgenerally in, a predetermined direction by offsetting the compressionload axis relative to the structural member and load centrallongitudinal axis, as described herein. In example embodiments, suchoffset is in parallel.

While the present disclosure has been described with respect to alimited number of embodiments, those skilled in the art, having benefitof this disclosure, will appreciate that other embodiments andmodifications can be devised which do not materially depart from thescope of the invention as disclosed herein. All such embodiments andmodifications are intended to be included within the scope of thisdisclosure as defined in the following claims.

The invention claimed is:
 1. A method of tuning a structural membercomprising an eccentricity to a fuse load, the method comprising:subjecting the structural member to a compressive axial load bycompressive axial displacement; and stopping the compressive axialdisplacement when the compressive axial load drops to a level of thefuse load, wherein said structural member is tuned to said fuse loadwhen said compressive axial load has plastically deformed saidstructural member.
 2. A method of tuning a structural member to a fuseload, the method comprising: subjecting the structural member to a sideload sufficient to bend said structural member in a desired direction;subjecting the structural member to a compressive axial load bycompressive axial displacement; stopping the compressive axial load andholding the axial displacement, when the structural member hasplastically deformed at mid-span or proximate mid-span; removing theside load; continuing subjecting the structural member to a compressiveaxial load by compressive axial displacement; and stopping thecompressive axial displacement when the compressive axial load drops toa level of the desired fuse load, wherein said structural member istuned to said fuse load when said compressive axial load has furtherplastically deformed said structural member.
 3. The method of claim 2,wherein continuing subjecting the structural member to a compressiveaxial load comprises continuing subjecting the structural member to anincreasing compressive axial load.